Effect of phosphorus on the microstructure and stress rupture properties in an Fe-Ni-Cr base superalloy
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I.
INTRODUCTION
PHOSPHORUS is always classified as a detrimental impurity in superalloys,[1] whereas relatively little is known about its effects, and the research about it is not systematic. Recent studies[2–5] showed that phosphorus greatly influences the solidification path, lowers the final solidification temperature, and aggravates the solidification segregation of elements in superalloys. Based on these studies, there has been a proposition that phosphorus should be controlled as low as possible to get supreme quality superalloys. However, to control the phosphorus content to an extremely low level will make the production complicated and uneconomical, because phosphorus cannot be removed by vacuum melting.[1] And, in addition, it is now uncertain what the condition is when the materials are pure of phosphorus. Except for the effect of phosphorus on the nonequilibrium segregation formed during solidification, the equilibrium segregation, which is generally constricted in a very small thickness and forms highly concentrated thin zones along grain boundaries, will also take place and play an important role in the microstructure and mechanical properties in the application. It can influence the grain boundary properties, such as intergranular diffusion, precipitation, and cohesion, as well as the microstructure and strength of the materials. Therefore, more work should be carried out on these aspects to evaluate the comprehensive effects of phosphorus. An Fe-Ni-Cr base superalloy, GH761 wrought alloy, is designed to provide high room and elevated temperature strength. The alloy is mainly strengthened by g'-Ni3(Al,Ti) phase. Because of the high addition of titanium (3.5 wt pct), the segregation of this alloy is relatively high, that is, prone to form the h-Ni3Ti phase. In this article, GH761 alloy is selected and studied for the preceding results and to reveal the mechanism by which phosphorus influences the superalloys. W.R. SUN, Research Fellow, is with the Korea Institute of Machinery and Materials, Changwon, Korea. S.R. GUO, D.Z. LU, and Z.Q. HU are with the Institute of Metal Research, Academia Sinica, Shenyang 110015, China. Manuscript submitted January 3, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
MATERIALS AND EXPERIMENTAL PROCEDURE
The materials were prepared in a vacuum induction furnace using high-purity raw materials. To minimize the compositional variations among the heats, the master alloy was prepared, and its composition (wt pct) was C0.011, Cr12.89, Ni43.54, W3.12, Mo1.59, Al1.71, Ti3.64, B0.003, Si0.059, and S0.003. Then the master alloy was remelted to give four 10-kg ingots, which were added into different contents of phosphorus with the same content of carbon and boron. The remelting also makes the alloy much more homogeneous. The levels of carbon, boron, and phosphorus were analyzed and are listed in Table I. The ingots were forged into bars of 45-mm-square sections at 1120 7C and rolled at the same temperature into round rods 18 mm in diameter. These materials were then given
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